Update to raylib 6.0 + add purego support for desktop platforms

[JupiterRider]

Do not merge please. This PR is not completely finished, but ready for review and testing.

This solves #542 and make #361obsolete.

• Update c sources to raylib 6.0
• Add purego implementation by using libffi (supports MacOS, Windows, Linux and FreeBSD)
• embed shared libraries (purego version)
• Update purego and cgo implementations to raylib 6.0 (functions, types, constants, etc.)
• Make the API/signature ( cgo <-> purego) match -> purego misses Asset.Open, Asset.ReadDir and Asset.ReadFile
• Review doc comments ( diff between cgo <-> purego) and update to raylib 6.0
• Update examples -> examples/models/m3d_loading needs a rewrite because of raylib's new Model Animation System done with 1cd6f30
• Update README.md with new purego instructions